Best Macular Dystrophy (BMD) is an inherited autosomal dominant disorder that exhibits symptoms and histopathology reminiscent of age-related macular degeneration (AMD,) the leading cause of blindness in the western world. Best disease results from mutations in the VMD2 gene, encoding the protein bestrophin. The only fully penetrant symptom of Best disease is a depressed light peak (LP) in the electrooculogram (EOG) in the absence of abnormalities in the clinical electroretinogram (ERG). The LP is generated by a Ca++ dependent CI conductance across the basolateral plasma membrane of the RPE cell. Consistent with the origin of the LP, our data suggest that bestrophin, the protein product of the VMD2 gene, is a regulator of voltage dependence and response kinetics of L-type voltage gated Ca++ channels. Based on data obtained in the previously funded period, we hypothesize that bestrophin functions to regulate the gain of the LP, by regulating the Ca++ dependent stimulation of a CI conductance. The studies proposed herein are designed to investigate this novel hypothesis in the only system in which it can be properly studied; the RPE cell. The goals of this study are to determine how bestrophin is involved in regulating or generating the LP, to explain the physiological consequences of bestrophin dysfunction in the eye, and to identify potential avenues for therapeutic intervention aimed at reducing or eliminating the loss of vision associated with Best disease. This will be accomplished via 3 specific aims. In the first aim a rat model of BMD in which adenovirus is used to overexpress bestrophin mutants will be used to assess the effects of bestrophin mutations on the LP via DC-electroretinography. In specific aim 2 we will assess the electrophysiological and histopathological phenotype of mice in which the vmd2 gene has been "knocked-out", or in which BMD associated mutations have been "knocked-in". In the third specific aim we will use a novel human RPE culture system to perform studies in Ussing chambers to assess the cellular consequences of bestrophin dysfunction. [unreadable] [unreadable]